Why did Comet 17P/Holmes burst out?

0.15 MB·

by  W. J. Altenhoff

Astronomy & Astrophysics manuscript no. 0458-astroph January 20, 2009
(DOI: will be inserted by hand later)
Why did Comet 17P/Holmes burst out?
Nucleus splitting or delayed sublimation?
1 1 1 3 2 1
W. J. Altenhoff , E. Kreysa , K. M. Menten , A. Sievers , C. Thum , and A. Weiss
9
0
0 1 Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, 53121 Bonn, Germany
2
e-mail: [email protected]
n 2 IRAM,University campus, F 38406 St. Martin d’Her`es, France
a e-mail: [email protected]
J 3 IRAM,Pico Veleta, Granada, Spain

8 e-mail: [email protected]
1

Received ; accepted
]
P
E Abstract. Based on millimeter-wavelength continuum observations we suggest that the recent “spectacle” of
. comet 17P/Holmes can be explained by a thick, air-tight dust cover and the effects of H2O sublimation, which
h started when the comet arrived at the heliocentric distance ≤ 2.5 AU. The porous structure inside the nucleus
p
provided enough surface for additional sublimation, which eventually led to the break up of the dust cover
-
o and to the observed outburst. The magnitude of the particle burst can be explained by the energy provided
r by insolation, stored in the dust cover and the nucleus within the months before the outburst: the subliming
t
s surface within the nucleus is more than one order of magnitude larger than the geometric surface of the nucleus
a – possibly an indication of thelatter’s porous structure. Anothersurprise is that theabundanceratios of several
[
molecular species with respect to H2O are variable. During this apparition, comet Holmes lost about 3% of its

1 mass, corresponding toa “dirty ice” layer of 20 m.
v
9 Key words.comets: general – comets: individual: 17P/Holmes
3
7
2
1. Introduction Montaltoetal.(2008) reporta significantdisassembly
.
1 of the nucleus, not even excluding a complete disintegra-
0 Comet17P/Holmeswasserendipitouslydiscoveredduring tion.Earlier,Sekanina(1982)hadclassifiedtypes ofsplit-
9
an outbreak on 1892 November 6 by Holmes (reported ting comets: (a) single comets that break up into two or
0
: by Plummer 1893) while he was observing the nearby more,(b) cometsthatdisintegrateorsuddenly disappear,
v Andromeda galaxy (M31). Until early 1893 January, the (c) and those with a pancake-shaped companion nucleus
i
X comet faded from magnitude 4 to 9–10, after which a that disintegrates into microscopic dust grains. Recently,
r second eruption to ≈ 5 mag occurred. Obviously, this Sekanina (2008) summarized the optical observations of
a light curve is different in time dependence and amplitude 17P/Holmes and some other comets for comparison. All
from what was observed during the most recent appari- types of splitting comets start in his hypothesis with a
tion(2007/8).Aftertheearlyobservations,CometHolmes major outburst. The “megaburst” of 17P/Holmes is of
was lost for some years, but later on recovered as an al- type (c), starting with an exothermic reaction, resulting
most “dead” comet with magnitudes of ≈ 16–17 near ev- in a rapidly expanding cloud of microscopic dust parti-
ery perihelion. Whipple (1986) analyzedthe historic data cles.Butnotallmajoroutburstsendinsplitting:e.g.,the
againandexplainedthetwooutburstsbygrazingencoun- oneofcometHalleyon1991February12ata heliocentric
ters of a smallhypothetical satellite with the nucleus:the distance, r, of 14 AU (Sekanina et al. 1992).
first one on 1892 Nov. 4.6, 1892 and the second on 1893
January16.3.;eventhough these encounters couldnot be
2. The nucleus
confirmed, his review of the historic observations allows
this event to be discussed again in connection with the 2.1. Time line
latest outburst discussed in this paper.
The “engine” behind the cometary activity of Comet
Holmes is the production of gaseous water as described
W. J. Altenhoff by Delsemme (1982). Its production rate, Q(H2O), is a
2 Altenhoff et al.: Comet Holmes
function of heliocentric distance. It is ∝ 1/r2 for low val- Table 1. Flux densities S (250) at 250 GHz in a 11”
ν
ues of r, while for r ≥ 1.5 AU the dependence becomes beam of 17P/Holmes. ∆ and r are the comet’s distance
highly nonlinear. Delsemme defines the limit of sublima- from Earth and Sun at time T after the outbreak.
tion r , the heliocentric distance beyond which 97.5% of
0
the energy received by insolation is re-radiated, and only date T ∆ r Sν(250) ref.
days AU AU mJy
≤ 2.5 % are used for vaporization. For water ice, r is
0
about 2.5 AU. Oct.27.105 3.3 1.630 2.447 64.5±2.8 (1)
28.205 4.4 1.628 2.451 55.5 2.6 (2)
Comet Holmes is a short-period comet in the Jupiter
Nov.16.950 24.2 1.634 2.530 16.0 4.3 (3)
family(aJFC).Itsaverageperiheliondistance,q,overthe
18.769 26.0 1.639 2.538 11.6 0.9 (3)
last 6 apparitions was ≈ 2.2 AU (Marsden and Williams
20.846 28.0 1.645 2.546 5.1 1.1 (3)
1999), close to the limit of H2O sublimation. But the 23.998 31.2 1.657 2.558 5.9 0.7 (3)
perihelion distance of the most recent apparition was 25.929 33.1 1.665 2.566 7.1 1.3 (3)
at a q of 2.05 AU. With the steepened production rate, 28.142 35.3 1.675 2.575 4.5 1.1 (3)
mentioned above, the H O production is increased by a Dec. 03.838 41.0 1.710 2.599 5.5 1.1 (3)
2
factor of 2. This is possibly responsible for the outburst. 18.333 55.5 1.850 2.670 4.1 1.5 (3)
(1) extrapolatedfromthefluxof2.3±0.1mJyobservedat88.6GHzby
The outburst happened according to Hsieh et al. Boissierelal.(seeSect.3)
(2) extrapolatedfrom2.1±0.1mJyobservedat90.6GHz(ibid.)
(2007)on2007October23.8,173daysafterperihelionpas-
(3) thispaper
sage,or361daysaftercrossingr=2.5AU.Probably–be-
causeofthe low-levelcometaryactivity –the nuclearsur-
facewasfreeoficeandtheicynucleuswascoveredbysome
H , of 16.6 (Tankredi 2006) was found and a median nu-
N
sort of a rubble pile (Jewitt, 1992) or dust-particle man-
clear diameter, d , of 3.2 km derived within the limits
N
tle, causing the delay of visible cometary sublimation by
of 4.6 and 2.9 km, corresponding to the albedo range.
months.Duringthisperiod,thedustcoverwas“airtight”,
Meanwhile, Lamy et al. (2005) report a diameter d =
N
preventing the sublimated gas to escape. Sublimation in-
3.42 km, obtained by a single snapshot by the Hubble
side the nucleus continued until the gas set free by this
Space Telescope (HST). We prefer this direct measure-
process broke up the dust mantle – the “outburst”.
ment, even though it might need a correction, if the nu-
cleus is not spherical.
2.2. Model parameter Bulk density, porosity. The bulk density may change
from comet to comet, depending, e.g., on the outgassing
Dustcover.Allaccuratelymeasuredcometarynucleishow history. For our model the value ρ = 0.5 g cm−3 was se-
a geometric albedo, p between 0.02 and 0.05 (see e.g. lected, derived by Rickman (1989) for the comet Halley
Jewitt2005),whichisindicativeofadustcover.Ifclosely data and from observations of 29 short period comets by
packed, this material is thought to have a density, ρ, ≈ Rickman et al. (1987). For the porosity (fraction of void
1 g cm−3, as inferred from numerous radar observations volume/bulk volume) we assume a value 0.60. This pro-
(Harmon1999;Harmonetal.2005).Informationfromthe vides ample storage for sublimated molecular gas inside
collisioncausedbytheDeepImpactmissionrevealedthat the nucleus.
the nucleus of comet Tempel 1 had a devolatilized dust Equilibrium temperature. One needs to know the
cover of about 1 m, with very little H O inside and none brightness temperature, T , of the nucleus and the dust
2 b
on the outside (Sunshine et al. 2007) – identical to what grains to calculate their emission. In the absence of new
weassumefor17P/Holmes.Belowthedustcoverofcomet data, we assume that they both will be close to the equi-
Tempel 1, an at least 10 m thick layer of fine grained wa- librium temperature, T . For a heliocentric distance of r
eq
ter ice particles was found, which appeared to be free of ≈2.45AUandanalbedopof0.04,weassumeT ≈175K,
b
refractoryimpurities!This“clean”icemayoriginatefrom i.e., identical to T .
eq
repeated sublimation and deposition inside the enclosed
nucleus approaching and leaving the solar neighborhood.
3. Observations
It is likely that comet Holmes has a similar layer.But for
our model we neglect this detail and assume for the mass The outburst of comet 17P/Holmes came at an unfa-
estimates “dirty” amorphous H2O ice throughoutthe nu- vorable moment, when on Pico Veleta the MAx-Planck
cleus. Millimeter BOlometer array (MAMBO) had not been in-
Diameter.Untilrecently,theresolutionofopticaltele- stalledonthe30mtelescope;inEffelsbergthe9mmwave-
scopes was not high enough to directly measure the nu- lengthreceiverwasnotoperationalatthe100mtelescope;
clear diameters of JFCs. Instead, absolute magnitudes of and for the Atacama Pathfinder EXperiment (APEX)
the nuclei were determined and nuclear diameters were 12mtelescope,the cometwasbelow the declinationlimit.
calculated,assumingageometricalbedop=0.04,because About 25dayslater,whenMAMBOwentbackinto oper-
observations constrainthe albedo to 0.02 < p ≤ 0.05 (see ation, we started a series of maps and ON/OFF observa-
e.g.Jewitt2005).For17P/Holmesanabsolutemagnitude, tionsatitseffectivefrequencyof250GHz,tryingtoseethe
Altenhoff et al.: Comet Holmes 3
aftermath of the outburst. The observing and evaluation
proceduresofMAMBOobservationsarestandardroutines
andhavebeenfrequentlyreported;seee.g.Altenhoffetal.
(2000).
The results are collected in Table 1.Priorto our mea-
surements, the comet had already been detected with the
Plateau de Bure Interferometer (PdBI) near 90 GHz by
Boissieretal.(2008,2009).Theirresults,generouslymade
available to us prior to publication, are included in our
analysis. We have scaled the 90 GHz flux densities, ob-
tainedwithasynthesizedbeamof5.7×7.3arcsec,tothe
angular resolution of our MAMBO data (11 arcsec), and
we extrapolatedthe signalto 250GHz withthe canonical
spectralindexofcometsSI=2.7,reversingtheprocedure
ofJewittandMatthews(1999)toderivethespectralindex
of comet Hale-Bopp. This method was intensively tested Fig.1.Comet17P/Holmes:Compilationofspectroscopic
by Altenhoff et al. (2008). andcontinuumobservations.Theblackdotsandtheblack
Each stage of the optical development has an equiva- diamonds represent the mm continuum data at 250 GHz,
lent one at millimeter (mm) wavelengths.The optical ob- taken with the 30m and the PdBI, respectively. Dashed
servations are summarized in Sekanina (2008), e.g. with line: model of mm halo (see text). Red open circles: H O
2
the total magnitude m as a function of time, “the light emission, observed with SWAN, light blue dots: HCN
1
curve” . emission, blue squares: HCN emission. See text for refer-
The mm data are compiled in Table 1 and Fig.1. The ences.Thedottedmagentacurveshowstheopticalnuclear
extrapolationofthePdBIobservationsto250GHzisfairly magnitudesm ,asanindicatorofthenuclearactivity.The
2
accurate, and the combined errors of extrapolation and spectroscopic data sets are normalized to their respective
observation are indicated by the size of the symbols. The maximum.
smallbeambroadeningbythecomet,reportedbyBoissier
et al. (2008) shows that the source is optically thin. The
resolutions. Even though we guess that, with the result-
two data sets are interpolated, suggesting a signal loss of
ing smearing,the productionratemightfitevenbetter to
7 % per day. The series of nuclear magnitudes m shows
2
our observed extended cometary activity.
a similar slope.
Spectroscopic observations of HCN by Biver et al.
In a separate paper, Altenhoff et al. (2008) show that
(2008) at Pico Veleta and at the Caltech Submillimeter
most cometary mm/radio light curves can be represented
Observatory (CSO), and by Drahus et al. (2007,2008)
by the following equation:
with the Arizona Radio Observatory (ARO), are shown
for comparison. The data sets are consistent which each
S =S ∆−2×r−1.7
ν ν,0 otherandshowasteeperdecaythanthecometaryactivity
with ∆ and r the geocentric and heliocentric distances in described before.
AU,respectively.The constantS = 74.5is derivedfrom
ν,0
the last data points.
4. Mass determination
Thus the light curve is calculated and plotted in Fig.
1. It is obviously a reasonable fit for the time after day Fine dust. Optically, the scattered light by small dust
33,wheninsolationanddustproduction(determiningthe particles is dominating the appearance of comets, even
intensity of the mm radiation) are apparently coming to thoughthe massofthese particlesis low.Sekanina(1982)
equilibrium. For the first 30 days, this radio light curve has estimated the mass of 2 µm sized fine dust in comet
is the baseline for the burst. As a further indicator of 17P/Holmes (see Table 2) near its outbreak. The size of
cometary activity, we use the nuclear magnitude, m , re- thescatteringparticlesistoosmalltodetectwithradioor
2
ported with the astrometric positions (Marsden 2007). mmtelescopes.Thisdustisresponsiblefortheopticalap-
Thesevalueswithlimitedaccuracyareaveragedoverthree pearanceseenatmagnitudem .Theparticulatedustand
1
days(typicallyover100observations)toreducethenoise. thebulkofthemoleculargasarealmostinvisibleoptically.
These data also confirm increased nuclear activity in the Particulate dust. Radio and mm continuum observa-
first 30 days. Red circles show the H O production rates tionsmeasurethethermalemissionofdustparticlesofsize
2
measured with the Solar Wind ANisotropy (SWAN) ex- ≥10%oftheobservingwavelength,here≥0.2mm.Since
periment on the SOlar Heliospheric Observatory(SOHO) the observed signal is proportional to the integrated par-
reported by Combi (2007). This system has a beam of ticle cross sections, but the particle mass is proportional
about one degree, probing the water production of about to its volume,the massofbigparticlesis underestimated,
4 days. This may be a crude guess, considering that we so observations at different wavelengths are needed for a
are using observing results obtained with very different more precise mass estimate.. We estimate the dust mass
4 Altenhoff et al.: Comet Holmes
with the photometric diameter to be the size of a disk cometarywindtostartthroughthedustmantleandlifting
at the distance of the comet with its equilibrium tem- dust particles,piece by piece, into the halo.The break up
perature, radiating as black body, yielding the same flux of the dust mantle is hardly spectacular, compared with
densityasthe radio/mmhalo.Forcometarydust,wefind the full start of cometary wind.
that the black body condition (emissivity ≈1) is fulfilled Time scale of the outburst. Different versions exist of
with a density of 1 g cm−3 and a layer depth of 3 wave- the development of the outburst. Sekanina (2008, 2007)
lengths (as confirmed by the rigorous halo evaluation for refers to an explosion and a single exothermic event,
comets HyakutakeandHale-Bopp(Altenhoff etal.2000). and Biver et al. (2008) and others report a water pro-
This allows calculation of the dust mass in the halo for duction rate, which almost ends after 3 days. In Fig. 1
any observed signal. the continuum observations are plotted, showing that the
Dust production rate. The average particle moves outburst-related increased continuum emission lasted for
through the telescope’s diffraction beam in about 60 about 30 days, as did the increased nuclear magnitude
hours,andtheresultingdustproductionrateandthedust m .Additionalproofarethenumerousphotographstaken
2
mass in the beam are listed in Table 2. within the first month of the outburst; see e.g. Sekanina
Hypothetical pre-burst dust. The radio light curve, as (2008), in which the comet appears as a filled Plerion
defined above, can be extrapolated backwards over the rather than a shell, implying that the dust injection into
whole apparition to calculate “hypothetical” signals and the coma continued after the “explosion” for quite some
masses that would have been emitted in the absence of time.
the dust cover. This total hypothetical mass is a factor Molecular production rates. Production of gas-phase
3 – 4 higher than the total mass released within 33 days molecules is responsiblefor all the cometaryactivity.It is
after the outburst, i.e. the burst dust mass. Thus, we can predominantly the cometary wind of the H O molecules,
2
safely assume that the insolation provided enough energy which lifts the dust particles from the nucleus, so a cor-
to start sublimation within the nucleus. relation between H O and dust production is expected.
2
Accuracy estimate. Within our observing interval, the Usuallytheproductionofdifferentmoleculesshowsafixed
mass in the halo is approximatelyproportionalto the ob- ratio, so that one can, e.g., predict the H O production
2
served flux density in the beam, thus to the observing rate from HCN observations. Not so for comet Holmes!
accuracy.Thereforethe relativeaccuracyfromdayto day Figre 1 shows that H O production, observed with the
2
and of the dust production rate is quite good. The abso- SWAN satellite, lasts at least for a month as does the en-
luteaccuracydependsonourknowledgeoftheabsorption hanced mm continuum emission, while e.g. the spectral
coefficient κ of cometary dust, whose uncertainty was es- lines of HCN, CO, NH (Drahus et al. 2007; Biver et al.
3
timated by Altenhoff et al. (2000) to be about a factor 2. 2008; Menten, 2007) had a big signal at the start, which
The accuracies of the mass determinations of the small- apparentlypeteredoutdramaticallyafter3days,asshown
grained dust (Sekanina 2007) and of water (Combi 2007) inFig.1.Thereasonmaybethetemperature/depthstruc-
have unfortunately not been reported. ture of the nucleus, because the near surface ice might be
free of volatile molecules.
Mass comparison. All derived masses are collected in
5. Interpretation
Table 2, where the total mass and the mass of the dust
Start of the outburst. The nuclear structure of comets cover have been calculated with the model values. The
9P/Tempel 1 and 17P/Holmes before the outbursts are mass of the 2 µm sized dust, determined immediately af-
probably alike, a densely packed dust cover (≈1 m) be- ter the outburst by Sekanina (2008), is surprisingly high,
low a layer of pure water ice (≈10 m), below amorphous compared to the total mass of the dust layer! Even the
dirty H O ice, whose upper part is possibly free of highly total particulate dust mass (grains of size 0.2 to 7 mm) is
2
volatile molecules. At 9P/Tempel 1 the impactor acted smaller.Integratedoverthe33daysofincreasedcometary
as the exothermic energy source to blow off the pancake- activity, the dust mass produced by the outburst is ≈ 2%
shaped dust cover, as the scheme of Sekanina (2007) sug- ofthecomet’stotalmass.SurprisinglylowisalsotheH O
2
gests, making it a type (c) split nucleus. The develop- mass released in the first part of the outburst, when we
mentfor17P/Holmesisdifferent.WhenH Osublimation would have expected a mass comparable to the particu-
2
started inside the porous nucleus, water vapor spread all late dust mass. The hypothetical dust mass, calculated
over the nucleus, initiating more sublimation; deeper in- from the radio light curve backwards,is about a factor of
side,andevenothermoleculariceswithlowersublimation 3 – 4 higher than the total mass in the burst. The energy
points were heated, stored there at lower temperatures. released in the burst can be provided by the insolation
The effective sublimating surface inside the nucleus, es- before the burst, even allowing energy losses through re-
timated as excess over the emission after the burst on radiation by the dust cover.
day 35 when it was near equilibrium with insolation, was The total accounted mass loss during this apparition
more than 14 times the nuclear surface, corresponding (mass of 2 µm sized dust, burst dust mass, burst H O
2
roughly to the nuclear size of comet Hale-Bopp. The sum mass) is in total ≤ 3.5% of the total nuclear mass. If a
ofthesaturatedpartialpressuresofallicespecieswasob- bulk density of ρ = 0.5 is assumed for the outer nucleus,
viouslybreakinguptheairtightdustmantle,allowingthe this loss correspondsto a layerof 20 m thickness which is
Altenhoff et al.: Comet Holmes 5
Table 2. Mass budget Biver, N., Bockel´ee-Morvan, D., Wiesemeyer, H., et al.,
Asteroids, Comets, Meteors 2008 held July 14-18, 2008 in
Contribution Mass Comment Baltimore, Md. LPI Contribution No. 1405, paper 8146
nuclear mass 1.11016g Model, ρ = 0.5 Boissier, J., Bockel´ee-Morvan, D., Biver, N., et al., Asteroids,
initial dust cover 3.71013g Model, ρ = 1.0 Comets, Meteors 2008 held July 14-18, 2008 in Balimore,
dust (∼2µm) 11014g (1) Md. LPI Contribution No. 1405. paper 8081
dust mass in halo 4.31013g on day 3 Boissier, J., Bockel´ee-Morvan, D., Biver, N., et al., 2009, in
dust production rate 2.0108g/s for day 3 preparation
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H2O production rate 3.6107g/s (2) Telegrams, Cambridge, USA
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Sekanina (2007,1982) explained the outburst of comet
Harmon,J.K.,Campbell,D.B.,Ostro,S.J.,&Nolan,M.C.,
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2
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Jewitt, D. C., and Matthews, H., 1999, AJ 117, 1056
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show several similarities to the present one, suggesting
and N. Thomas, Springer-Verlag, Berlin p. 1 - 78
that they happened the same way, but in 2 steps. After
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all,comet17P/Holmesisacometlikemanyotherswhose
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L45 - L49
ful to Dr. J. Boissier (IRAM) for communicating the 90
Plummer, W.E., Report of theCouncil, 1893, MN 53, 266
GHz results to us prior to publication. We thank the di-
Rickman,H.,Kamel,L.,Festou,M.C.,andFroeschl´e,C.,1987.
rectorofIRAM,Dr.P.Cox,forgrantingspecialobserving In:Symposiumon theDiversityandSimilarity ofComets.
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